The so-called sunshade films or reflection films that are applied for protection against sunlight on single-glazed windows on the inside glass surface and are likewise provided on the inner side of the outer pane of bonded double glazing or the inside of the outer pane of spaced insulating double glazing, in each case by means of an adhesive, are known protective barriers of the general type described above. Such reflecting films use a partly transparent polyester film as the synthetic resin layer that carries on one side a vapor deposit metal film, as a rule of aluminum. The metal film is covered by a cover layer on the side of it facing away from the polyester film, the cover layer as a rule consisting of another polyester film. Both polyester films in addition to functioning as supporting carriers, provide protection against corrosion and scratching up of the metal film. Such sunlight protection films affixed to glass panes or panels by adhesive are intended to reduce the radiation to the room of heat rays, glare and ultraviolet rays of the sun.
The sunlight-protection films, on the other hand, are not suited to reduce the passage of heat in the opposite direction, i.e. from the room space out to the exterior. This is to be explained by the high absorptivity of the synthetic resin layer, in the form of a polyester film, which is provided on the room side of the material. This polyester film warms up under the influence of heat radiation of any source of heat located in the room. As the result of heat conduction, this heat is transmitted through the metal film to the cover layer, which is another polyester film and from there it is radiated away without undergoing any reflection by the metal film. Only a small part of the secondary heat radiation produced by heating up of the material is regained by radiation back into the room space, because it is transmitted back only according to the comparison of the transition resistances on the room side and the external side. In consequence, such known reflection films are limited to the sunlight protection service alone and make possible practically no appreciable heat loss protection of the room space against the exterior. There is a further disadvantage that the vapor deposited metal film must be unusually uniform, which naturally raises the costs involved in the production of the material to a high level. Such sunlight protection films are therefore of rather high price. Their application by use of an adhesive requires about twice their price per square meter, and furthermore, must be performed exclusively by specialized craftsmen.
There is also known a kind of double glazing in which the inner surface of the outer pane has a thin vapor deposited gold film. The inner pane provides a relatively good protection of the metal film against the mechanical damage. The metal film is partly transparent in the visible light spectral region, with a color-changing effect, but reflects at the same time large proportions of the incident sunlight and thus offers a very good protection against sunlight. Furthermore, the metal film also reflects the largest portion of the long wave infra-red radiation from the room space and thus reduces the heat transmissivity. The vapor deposited gold filmed double glazing is used predominantly as a sunlight protection measure in the construction of office buildings. It has the disadvantage that its price per square meter is above five times that of the just described sunlight protection films to be applied by adhesion to the window surfaces. Therefore, for economic reasons, the installation of gold-filmed insulating windows is considered for only a limited range of uses, thus in no case in the field of private dwellings or public buildings.